The invention is related to the domain of the “medical devices” as defined by the directive 93/42/CE of Jun. 14, 1993 of the European Communities, and notably the “active implantable medical devices” as defined by the directive 90/385/CEE of Jun. 20, 1990 of the European Communities. This definition in particular includes the implants that continuously monitor the cardiac rhythm and deliver if necessary to the heart electrical pulses of stimulation, cardiac resynchronization, cardioversion and/or defibrillation in case of a rhythm disorder detected by the device. It also includes neurological devices, cochlear implants, etc., as well as devices for pH measurement or devices for intracorporeal impedance measurement (such as the measure of the transpulmonary impedance or of the intracardiac impedance).
The invention relates more particularly to those of these devices that implement autonomous implanted capsules and are free from any physical connection to a main implanted (such as the can of a stimulation pulse generator).
These autonomous capsules are called for this reason “leadless capsules” to distinguish them from the electrodes or sensors placed at the distal end of a lead, this lead being traversed throughout its length by one or more conductors connecting by galvanic liaison the electrode or the sensor to a generator connected at the opposite, proximal end, of the lead. Such leadless capsules are for example described in U.S. 2007/0088397 A1 and WO 2007/047681 A2 (Nanostim, Inc.) or in the U.S. 2006/0136004 A1 (EBR Systems, Inc.).
These leadless capsules can be epicardial capsules, fixed to the outer wall of the heart, or endocardial capsules, fixed to the inside wall of a ventricular or atrial cavity, by a protruding anchoring helical screw, axially extending the body of the capsule and designed to penetrate the heart tissue by screwing to the implantation site. The invention is nevertheless not limited to a particular type of capsule, and is equally applicable to any type of leadless capsule, regardless of its functional purpose.
A leadless capsule includes various electronic circuits, sensors, etc., and a transmitter/receiver for wireless communication for remote data exchange. The signal processing inside the capsule and its remote transmission requires a non-negligible energy compared to the energy resources this capsule can store. However, due to its autonomous nature, the capsule can only use its own resources, such as an energy harvester circuit (by the movement of the capsule), associated with an integrated small buffer battery.
The invention relates more particularly to capsules whose energy harvesting uses a mechanic-electric transducer of the piezoelectric type (hereinafter “piezoelectric component”) cyclically and alternatively stressed in bending so as to generate electric charges, which charges are then harvested by the self-supply of the capsule system. The mechanical stress of the component can in particular be caused by variations in the pressure of fluid surrounding the capsule (typically, the blood medium), which cyclically deforms or moves a flexible membrane or a mobile surface linked to a bellows (elements designated hereinafter as “actuation element”), this membrane or this surface being connected to the piezoelectric component by a suitable coupling element such as a rod, a strut, etc. (hereinafter “connection element”). EP 2639845 A1 (Sorin CRM) describes such a structure of energy harvesting. Other examples of energy harvesters implementing a piezoelectric component are disclosed by WO 2013/081560 A1, U.S. Pat. No. 3,456,134 A, US 2012/286625 A1, WO 2013/121759 A1 or WO 2013/077301 A1.
Two configurations are possible, depending on the method by which the piezoelectric component is mounted in the casing of the capsule and according to the position of the point of application of the force transmitted by the connection element which stresses the component.
In a first, not symmetrical, configuration the piezoelectric component is blade-shaped or beam-shaped (in the sense of strength of materials) secured to the body of the capsule at one of its ends (“free-clamped” configuration) and is stressed in bending by a force applied to its free opposite end. This configuration allows a maximum deformation of the blade, so a high level of charges is generated and thus provides efficient mechanic-electric conversion. However, the non-symmetrical arrangement of the various elements, in particular of the connection element relative to the body, requires a non-symmetrical displacement of the actuation element and a non-homogeneous deformation of the diaphragm or of the bellows relative to the body of the capsule, which is undesirable for mechanical reliability reasons.
In a second, symmetric, configuration the piezoelectric component is secured to the body at its two ends (“clamped-clamped” configuration) and subjected to bending stress by a force applied to its center. This configuration allows movement of the operation element parallel to itself and therefore a homogeneous deformation of the diaphragm or bellows. However, it does not allow a high amplitude of deformation of the piezoelectric component, said second configuration being more rigid than the first. Typically, for a given dimension of the component, the stiffness of a clamped-clamped configuration is eight times higher than that of a clamped-free configuration, requiring a bending displacement of the component, and thus to eight times less of harvested energy.
To increase flexibility, one must either reduce the thickness of the piezoelectric component (but the limits of this technological solution is quickly reached) or increase its length while maintaining the symmetrical configuration. This solution is proposed by the EP 2 639 845 cited above, which teaches structuring the component with a spiral or coil shape to increase the effective length and flexibility, while maintaining a centered coupling allowing symmetric deformation of the bellows or diaphragm. However, although such structures are very flexible, transduction performance remains relatively low, due to two specific phenomena:    Curved or wound structures are subject to phenomena of torsion, which consequently results in a large part of the mechanical energy applied to the transducer stored as torsion elastic energy, while only the bending energy is converted into electricity; and    Mechanical deformations of an elongated and wound structure (even a simple linear structure of the clamped-clamped type) are complex, with curvature inflections. Under stress, the component has zones under tension alternating with zones under compression, creating changes of sign of the electrical potential created by the piezoelectric effect (the more elongated and complex the structure is to increase its flexibility the more changes). This phenomenon can be taken into account by providing the component with charge harvesting electrodes which are separate for each respective zone stressed in tension or compression. However, the structuring of the electrodes adds additional complexity of design and realization of the component, without completely solving the problem of the poor conversion efficiency resulting from the multiplication of sign changes of the electric potential in the piezoelectric material.